The overall goal of this renewal application by a group of clinicians and basic scientists is to elucidate the fundamental mechanisms in electrical stimulation of the auditory nerve, and to provide a rational foundation for the design of better cochlear prostheses to alleviate speech reception deficits in the profoundly deaf. During the current grant period, psychophysical, speech-perception, speech-production, evoked-potential, and anatomical (CT) data have been systematically collected for 21 implanted patients, some of whom have been studied longitudinally for more than 4 years. This clinical effort and our related animal studies are beginning to identify basic factors underlying the wide variability in benefits obtained by implantees. Although the proposed research is highly interactive, it is organized into 5 projects according to methodology: (1) Studies of human perception to characterize speech reception in implanted subjects and relate it to information processing capabilities of the subjects and to speech processing schemes. (2) Studies of speech production to characterize changes that occur in the speech of implanted subjects and to understand the role of self hearing in speech. (3) Evaluation of electrically-evoked potentials to determine their utility for adjusting processors for patients who cannot provide reliable behavioral responses (e.g. infants), and as a means of estimating the number and distribution of spiral ganglion cell populations. (4) Anatomical studies using CT scans and histological techniques to investigate the morphology of temporal bone structures important for cochlear implantation and to determine electrode locations after implantation. (5) Electrophysiological experiments in cats to describe the responses of auditory nerve fibers to intracochlear electrical stimulation in order to describe how well speech information is encoded by existing speech processors with the aim of improving those processors. These projects interact with a biophysical model for electrical stimulation of myelinated auditory nerve fibers that will be coupled to an electro-anatomical model of current flow in the cochlea; they provide the data necessary for testing the models and the models provides a unifying theoretical basis to aid in the interpretation of results.